This invention relates to an apparatus for converting energy to some useful form and particularly to an apparatus and method for energy conversion using gas hydrates.
Electrical power is commonly produced using a high temperature and high pressure gas, such as super heated steam, to drive a turbine. The turbine then is used to drive a generator to produce electricity. Most of the cost of producing electricity is in the fuel used to produce the high temperature and high pressure gas for driving the turbine. Alternatively to burning a fuel to produce steam, the turbine can be driven directly by the combustion products of the fuel. In either case, the system converts energy from a compressed gas using the turbine or similar device.
Gas hydrates, hereafter sometimes referred to simply as "hydrates", are crystalline structures of water and a host gas molecule. Numerous different types of gases form hydrates, including methane and nitrogen. As used in this disclosure, "working gas" means any gas or mixture of gases capable of forming gas hydrate with water. The gas hydrate crystal can have one of two forms, each having a geometrically figured cell made up of a number of water molecules and a number of cavities. Structure I is a body centered cubic cell containing 46 water molecules per unit cell with 3 small cavities and 6 large cavities. Structure II is a diamond lattice cell containing 136 water molecules per unit cell with 16 small cavities and 8 large cavities. The cavities are filled with a host gas or gases such as hydrocarbon gases in order to stabilize the molecules. In hydrates formed with hydrocarbon host gases, only methane fills the small cavities while methane, ethane, propane, i-butane, n-butane, and n-pentane fill the large cavities.
Gas hydrates have an unusual vapor pressure-temperature relationship in that the vapor pressure increases exponentially with a linear increase in temperature. For example, pure methane hydrate has a vapor pressure of 410 psia at 35.degree. F. and a vapor pressure of 2,043 psia at 62.degree. F. Thus, at a compression ratio of 4, the temperature of the gas forming the hydrate is only increased by 27.degree. F. By contrast, a normal adiabatic compressor would increase gas temperature by 225.degree. F. in order to achieve a compression ratio of 4. This increased temperature in adiabatic compression represents wasted energy not necessary to compress the gas.
The invention disclosed herein takes advantage of this unique property of gas hydrates to provide near-isothermal compression of gases. The gas may be used to drive a turbine or other device in order to convert energy from the compressed gas to electricity, for example.